Various membrane systems can be used to separate chemicals of different species. Often they are used to remove water from a dilute solution to produce a concentrated product. Sometimes other species are removed with the water.
Reverse Osmosis (RO) is a crossflow membrane technology that was developed more than 35 years ago. A typical membrane is synthetic polymer coated onto a backing material so as to create pores that are only a few Angstoms in diameter. Purified water passes through the membrane and other contaminants are rejected or retained by the membrane. Membrane materials include cellulose acetate (CA), polyamide (PA), and sulfonated polysulfone (SPS).
The typical RO system feeds impure water at high pressure to the one side of the membrane and removes purified water from the other side of the membrane at low pressure, near atmospheric. The greater the driving force, the better the flow of water and the greater the separation effectiveness of salts such as sodium chloride from the water. The high pressure side product is thus depleted of water, and if left containing a more concentrated solution of the salts. This is called the retentate. The low pressure side contains the purified water and is called the permeate.
As the salt is rejected by the RO membrane, the osmotic pressure on the retentate side increases. This means that the water is increasingly retained on the retentate side by attraction to the salt species. Thus it is often necessary and desirable to use high pressures on the retentate side and low pressures on the permeate side to allow greater concentration of the retentate. In addition, the greater the trans-membrane pressure, the higher the flux.
Often RO membranes are designed to try to reject all salts and organic species except water. A typical water purification system may run at 400-600 psig on the retentate side and 5-10 psig on the permeate side. For example, hollow fibre membrane bundles use small fibres to withstand the high retentate side pressure and an economical plastic housing on the permeate side to collect the purified water.
Nanofiltration (NF), ultra-osmosis (UO), and loose reverse osmosis (loose RO) are all terms used to describe membranes that permeate some low molecular weight materials or salts while retaining others. The present description and specification will use NF to represent all of these processes. They are very similiar to RO. Some common NF membranes are used to permeate water and sodium ions and chloride ions, while rejecting (and thus retaining) calcium salts and also retaining sugars such as lactose.
Organic acid recovery from salts is necessary for purification of fermentation broths, neutralized wood processing wastes for example such as those containing sodium acetate, and memory other organic acid streams. Organic acids are present in streams such as fermentation broths, pulping waste streams, citrus processing waste streams. This invention applies to streams containing salts of organic acids that are in the pH range 3 to 8.
Acids by fermentation can be produced by the continuous or batch fermentation of sugars or other biomass streams such as hydrolysed starch, sulfite waste liquor, or cheese whey. For example lactic acid can be produced by the continuous or batch fermentation of sugars or other biomass streams such as hydrolysed starch, sulfite waste liquor, or cheese whey. For a rapid and economic fermentation, the pH of the broth is usually maintained in the rage of 4.5 to 7.5 by either
(a) continuously removing lactic acid such as by extraction, or membranes, or ion exchange, or electrodialysis, or
(b) continuously adding a base such as aqueous ammonia, calcium carbonate, calcium hydroxide, or sodium hydroxide, or
(c) starting the fermentation with a growth medium with substantial buffering capacity, such as a calcium carbonate slurry.
In case (a) the fermentation is integrated with the first step of the product recovery process. At a pH of 5 to 7 at 30.degree.-50.degree. C., the lactic acid is present mainly as lactate salts rather than free acid.
In cases (b) and (c) and in some variants of case (a) above, the fermentation product is a crude aqueous broth at pH 4 to 8 containing 1 to 20% weight lactate salts, together with impurities such as medium components, neutralizing agent impurities, biomass, and salts of other acids. In each case, the fermentation broth may or may not have the fermentation micoorganisms or enzyme slurry removed by methods such as centrifugation, membranes processes, coagulation, or other methods, and this removal may take place during or after the production of the crude fermentation broth.
Other typical organic acids and microorganisms for their production that are disclosed by Atkinson and Mavituna (Biological Engineering and Biotechnology Handbook p 421 (1983)) Nature Press, are as follows:
______________________________________ One example of a microorganism Acid that can produce this acid ______________________________________ Acetic acid Acetobacter acetii L-allo-isocitric acid Penicillium purpurogenum beta-Arabo-ascorbic acid Penicillium notatum Citric acid Aspergillus niger Candida lipolytica Fumaric acid Rhizopus delemar Gluconic acid Aspergillus niger L-isocitric acid Candida brumptii Itaconic acid Aspergillus terreus 2-ketoglyconic acid Pseudomonas fluorescens 5-ketogluconic acid Gluconobacter suboxydans alpha-ketoglutaric acid Candida hydrocarbofumarica Kojic acid Aspergillus oryzae Lactic acid Lactobacillus delbruckeii Malic acid Lactobacillus brevis Propionic acid Propionibacterium shermanii Pyruvic acid Pseudomonas aeruginosa Succinic acid Bacterium succinicum Tartartic acid Gluconobacter suboxydans ______________________________________
This invention pertains to salts of organic acids including salts of aliphatic monocarboxylic acids containing 1 to 5 carbon atoms, e.g. formic acid, acetic acid, propionic acid, butyric acid, and pentanoic acid; salts of aliphatic alpha hydroxy monocarboxylic acids containing 2 to 4 carbon atoms, e.g. hydroxyacetic acid, lactic acid and alpha hydroxy-butyric acid; salts of aliphatic beta hydroxy monocarboxylic acids containing 3 or 4 carbon atoms, e.g. 1-hydroxy-3-propionic acid and beta hydroxy butytric acid; salts of olefinic monocarboxylic acids containing 3 to 4 carbon atoms such as acrylic acid and methacrlyic acid; and salts of dicarboxylic acids containing 3 to 4 carbon atoms such as succinic acid.
Some of the problems of organic acid recovery from salts include:
(a) Purity -- Often a high purity product is required. Many processes require many steps to achive this purity
(b) Thermal processing -- Any process that requires thermal processing as part of the purification sequence will likely generate undesirable reaction products and/or precipitates.
(c) Base recovery -- Many processes require addition directly or indirectly of a strong inorganic acid such as hydrochloric acid or sulfuric acid to release the organic acid from its salt. These processes generate a new dilute or concentrated solution containing the salt of the new inorganic acid. For example calcium lactate is contacted with sulfuric acid to release lactic acid and to produce calcium sulfate which precipitates. The calcium sulfate must then be removed and dealt with. Calcium hydroxide or calcium carbonate is not readily recovered for reuse from this process.
Electrodialysis can separate the organic acid salt into free acid and regenerated base which can be re-used for fermentation or other pH control needs. These processes require elaborate membrane systems and typically have significant electricity costs.
(d) Cost -- a simple, low temperature, solids-free process is likley to give the lowest cost.
(e) Complexity -- a simple process that leads directly to a high purity product is desirable.
Another important separation process involves weak organic base recovery. This is the inverse of the case of organic acid recovery. Instead of recovering the organic acid from its salt solution, recover the organic base from its salt solution is recovered. For example, consider the case of the recovery of amines used to scrub acid vapors. Here an amine such as ethylamine may be contacted with an acid such as HCL to scrub the acid from vapor. The resultant solution of the chloride salt of the amine must then be recovered and regenerated for re-use.
Still another important area is amino acid recovery. Amino acids such as lysine may be produced by fermentation. In these processes, the pH is often controlled to aid performance of the fermenting micro-organisms. Amino acids are called amphoteric, in that they have both acidic and basic groups. All amino acids have a common formula: EQU HOOC--CR.sub.1 R.sub.2 --NH.sub.2
where R.sub.1 and R.sub.2 are simple or complex side groups. If the pH of the broth is controlled with ammonia, then the resultant species in solution will be EQU [NH.sub.4 ]+--[OOC--R.sub.1 R.sub.2 --NH.sub.2]
Separation of the free amino acid from the salt is often done by various means including ion exchange. In the current invention, there is show and described a new way to recover amino acids.
Shimshick (1981) U.S. Pat. No. 4,250,331 reports a supercritical extraction-acidification process for recovering carboxylic acids from dilute aqueous solutions of alkali metal salts of such carboxylic acids. Feed solutions are mixed with 10 to 1000% of a supercritical solution comprising at least 10 mole carbon dioxide. The salt reacts with the carbon dioxide to form the carboxylic acid which dissolves in the supercrical fluid phase. The aqueous and supercritical phases are then separated. The pressure of the supercritical phase is then lowered so that an acid phase is formed separate from the supercritical phase, whereby the acid is recovered. This differs extensively from the current invention. The current invention does not use extraction, does not require use of supercritical conditions, but only requires use a membrane system.
Yates (1981) U.S. Pat. No. 4,282,323 reports a sub-critical extraction acidification process, very similiar to that of Shimshick. The key difference is that the Shimshick patent uses supercritical carbon dioxide as the extracting solvent, whereas the Yates patent uses a polar organic extracting with carbon dioxide under pressure.
Donohue et al (1989) report using a cellulose acetate membrane to separate carbon dioxide -- methane gas mixtures. They found that the permeability of carbon dioxide through the membrane increased dramatically with pressure. They believed that this was due to plasticization of the cellulose acetate by the carbon dioxide.
Awadalla et al (1994) used membranes to remove ammonium and other ionized species from mining waste water. They report that reverse osmosis membranes (the exact type was not specified) were found to provide good rejection of ammonium ion (&gt;99%) while poor rejection of free ammonia (10-30%). Here we see that a membrane system can be used to achieve a pH dependent separation.
Some membranes exhibit excellent rejection of free acid form of organic acids, and others exhibit poor rejection. Examples are
TABLE 1 ______________________________________ Examples of some membranes exhibiting excellent rejection (% rej) of free acids Feed conc Membrane Solute wt % % Rej ______________________________________ Polyamide (FT-30) acetic acid 0.07 92 Polyamide (FT-30) butyric acid 0.09 95 TFC (PCI ZF99) lactic acid 1.0 90-95% ______________________________________
TABLE 2 ______________________________________ Examples of some membranes exhibiting poor rejection (% rej) of free acids Feed conc Membrane Solute wt % % Rej ______________________________________ Cellulose acetate acetic acid 1.0 7 Cellulose acetate lactic acid 20-40% (CA-97) ______________________________________
TABLE 3 ______________________________________ Examples of membrane exhibiting poor rejection (% rej) of free base but good rejection of ionized base Methylamine removal ______________________________________ acidic solution 98% removal basic solution 50% removal ______________________________________ NS-100 membrane microporous polysulfone coated with polyethylenimine whic is crosslinked with mtolylene-2,4-diisocyante.
TABLE 4 ______________________________________ Literature Data for acetic acid/acetate and a NS-100 membrane. % Dissociation % Removal of acetate pH of acetate ______________________________________ .99 6.75 97% .80 5.36 80% .40 4.58 50% .20 4.15 42% .01 2.76 30% ______________________________________
TABLE 5 ______________________________________ Literature Data for separation of lactic acid from lactate using an HC-50 polyamide on polysulfone thin film composite membrane. 1% w/w lactic acid solution % dissociation % rejection pH of lactate of lactate ______________________________________ 6.00 99.9% 4.93 98.6% 80% 4.43 95.8% 72% 3.93 87.9% 60% 3.45 70.6% 43% 2.88 39.2% 32% 2.00 7.8% ______________________________________
TABLE 6 ______________________________________ Literature Data for separation of acetic acid from water and for separation of sodium acetate from water using a DuPont Hollow Fibre B-9 permeator, at 500-680 ppm feed concentration % dissociation % rejection pH of acetate of acetate ______________________________________ 8.1 99.95% 98% 3.7 8.06% 40% ______________________________________